Demands for a power MOSFET have increased in the market of switching power-supply devices with a large current and a high withstand voltage. The power MOSFET is used for a DC-DC converter or a synchronous rectifier.
JP-A-2013-69852 and US 2014/0117439 disclose a MOSFET with a trench MOS structure as a technique of reducing on-resistance of a power MOSFET. The MOSFET with a trench MOS structure has a structure in which an N-type semiconductor layer connected to the drain electrode, a P-type semiconductor layer forming a channel, and an N-type semiconductor layer connected to the source electrode are arranged in the depth direction of a semiconductor substrate, and the gate electrode is embedded to be adjacent to the P-type semiconductor layer, with an insulating layer interposed therebetween, in the semiconductor substrate.